Provide missing description for our FLEXIBLE_ARRAY_MEMBER defines
Apparently autoheader throws a tantrum if you say 'AC_DEFINE([a], [b])'. Instead you must say 'AC_DEFINE([a], [b], [description of a])'. We were running into this in our replacement definitions for FLEXIBLE_ARRAY_MEMBER, which were only getting built on autoconf versions before 2.61 -- and this made us stop working with those autoconf versinos. Fixes bug 2430; bugfix on 0.2.3.1-alpha.